1. Field of the Invention
The present invention relates to a light scanning device and an image forming apparatus using the same and, more particularly, to a light scanning device which is suitable for a laser beam printer or digital copying apparatus which has, for example, an electrophotographic process of reflecting/deflecting (deflecting/scanning) a light beam, optically modulated and emitted from a light source means, by using a deflecting element formed from a rotating polyhedral mirror or the like, and recording image information by optically scanning a surface to be scanned through an imaging optical system having f-xcex8 characteristics.
2. Description of Related Art
In a conventional scanning optical device for a laser beam printer (LBP) or the like, the light beam which is optically modulated in accordance with an image signal and emitted from a light source means is cyclically deflected by a light deflector formed from, e.g., a rotating polyhedral mirror (polygon mirror), and the light beam is focused into a spot on the surface of a photosensitive recording medium (photosensitive drum) and optically scanned on the surface by an imaging optical system having f-xcex8 characteristics.
FIG. 11 is a schematic view showing the main part of a conventional light scanning device.
Referring to FIG. 11, the divergent light beam emitted from a light source means 71 is converted into a substantially parallel light beam by a collimator lens 72, and the light beam is limited by a stop 73 to be incident on a cylindrical lens 74 having predetermined refracting power only in the sub scanning direction. Of the substantially parallel light beam incident on the cylindrical lens 74, the light in a main scanning section emerges without any change. The light in a sub-scanning section is focused and substantially formed into an almost line image on a deflecting surface (reflecting surface) 75a of a light deflector 75.
The light beam reflected/deflected by the deflecting surface 75a of the light deflector 75 is guided onto the photosensitive drum surface 78 serving as a surface to be scanned through an imaging optical system (f-xcex8 lens system) 76 having f-xcex8 characteristics. By rotating the light deflector 75 in the direction indicated by an arrow A, a photosensitive drum surface 78 is optically scanned to record image information.
In such a light scanning device, in order to record high-resolution image information, for example, the following requirements need to be satisfied. The curvature of the image surface is properly corrected throughout the surface to be scanned. Distortion characteristics (f-xcex8 characteristics) with uniform velocity characteristics are set between a scanning angle xcex8 and an image height in the Y direction. A spot diameter on the image surface at each image height is uniform. Conventionally, various types of light scanning devices that satisfy such optical characteristics or correction optical systems (f-xcex8 lenses) have been proposed.
As laser beam printers and digital copying apparatuses have decreased in size and cost, similar requirements are imposed on light scanning devices.
As an arrangement that satisfies such requirements, a light scanning device having an f-xcex8 lens formed from a single lens is disclosed in, for example, Japanese Patent Application Laid-Open Nos. 4-50908 and 9-33850.
According to Japanese Patent Application Laid-Open No. 4-50908, a high-order aspherical surface is used in the main scanning direction of an f-xcex8 lens to relatively properly correct aberration characteristics. However, since the magnification in the sub scanning direction between a light deflector and a surface to be scanned is not uniform, the spot diameter in the sub scanning direction tends to change with a change in image height.
According to Japanese Patent Application Laid-Open No. 9-33850, the curvatures of at least two of the lens surfaces of an f-xcex8 lens in the light scanning device in the sub scanning direction continuously change at the effective portion of an imaging lens along the main scanning direction independently of the curvature in the main scanning direction. This makes it possible to control the position of the principal plane in the sub scanning direction by bending the two surfaces and make the sub scanning magnification at each image height uniform, thereby making the spot diameter uniform.
In the above proposal, in order to make the sub scanning magnification uniform, the position of the principal plane is so controlled as to make the main scanning uniform by bending at least two surfaces. Although this allows completely independent setting of a main scanning shape and sub scanning shape, requirements such as suppression of an increase in lens thickness tend to make the lens shape in the main scanning direction have a relatively large aspherical surface amount.
A lens having a large aspherical surface amount in the main scanning direction like the one described above is subjected to considerable deteriorations in optical performance due to the arrangement errors of the respective lens surfaces and lens. Of the deteriorations in optical performance, scanning line bending in the sub scanning direction, in particular, cannot be corrected by adjusting mirrors and the like arranged in the device body, unlike scanning line height deviation, scanning line inclination, and the like. This therefore poses a serial problems. In order to suppress scanning line bending to a low level, the respective lens surfaces and the lens need to be arranged with high precision in accordance with design values or an adjusting mechanism needs to be provided for the lens to adjust the arrangement according to the design values.
FIG. 13 is a graph in which a scanning line bending amount is plotted at each image height, with an optical axis regarded as an origin, in the scanning lens disclosed as the first embodiment (FIG. 12) in Japanese Patent Application Laid-Open No. 9-33850 when each surface and the lens block are decentered by 50 xcexcm in the Z direction (a direction perpendicular to the optical axis and the main scanning direction). In FIG. 12, the reference characters S1 to S8 respectively identify a light source, a collimator lens incident surface (plane), a collimator lens exit surface, a cylinder lens incident surface (plane), a cylinder lens exit surface, a deflection surface, a scanning lens incident surface and a scanning lens exit surface. In FIGS. 13 and 14, Block, R1, R2 respectively refer to a lens, an incident surface and an exit surface. FIG. 14 shows the scanning line bending amounts in a case wherein each surface and the lens block are tilted by 3xe2x80x2 around a straight line (rotation axis) which passes through the respective surface vertexes (the surface vertex on the incident side of the lens in the case of the lens block) and is parallel to the main scanning direction.
In a scanning lens of a type which has a large aspherical surface amount in the main scanning direction and also has power in the sub scanning direction on the two surfaces, a very large scanning line bending amount is produced by decentering in the Z direction. In addition, the scanning line bend sensitivity level with respect to a tilt is high. Obviously, therefore, even if the performance of the lens is compensated in terms of design, the lens does not reach a practical level when it actually processed and mounted in a housing.
In a color image forming apparatus in which light scanning devices are respectively arranged in correspondence with four photosensitive devices (photosensitive drums), and latent images are formed on them by laser beams to form Y (yellow), M (magenta), C (cyan), and Bk (black) original images on the corresponding photosensitive device surfaces, since the four color images, i.e., the Y, M, C, and Bk images, formed on the respective photosensitive device surfaces are superimposed on a transfer medium such as a paper sheet, if scanning lines of the light scanning devices corresponding to the respective photosensitive devices bend, errors are produced in the shapes of the scanning lines among four colors. This causes color misregistration on the image on the transfer medium, and hence results in a considerable deterioration in image quality.
The present invention has been made to solve the above problems, and has as its object to provide a light scanning device which has an f-xcex8 lens formed from a single lens, and can suppress scanning line bending caused by the arrangement error of the lens to a low level by properly correcting image surface bending characteristics as characteristics of an optical system, f-xcex8 characteristics for uniform velocity scanning, and wave aberration and optimally shaping the f-xcex8 lens, and an image forming apparatus using the device.
An object of the present invention is to provide a light scanning device having an imaging optical system for guiding a light beam emitted from light source means to a deflecting element, and forming the light beam deflected by the deflecting element into an image on a surface to be scanned, characterized in that, the imaging optical system is formed from a single lens, sectional shapes of an incident surface and an exit surface of the single lens in a main scanning direction are non-arc shapes, power of the exit surface in a sub scanning direction satisfies 0.9xe2x89xa6xcfx86s2/xcfx86sxe2x89xa61.1 where xcfx86s is power of the overall imaging optical system in the sub scanning direction and xcfx86s2 is power of the exit surface in the sub scanning direction, and the non-arc shape of the exit surface in the main scanning direction satisfies       0.9    xc3x97                  L        bo                    L        ao              ≤            L              b        ⁢                  xe2x80x83                ⁢        θ                    L              a        ⁢                  xe2x80x83                ⁢        θ              ≤      1.1    xc3x97                  L        bo                    L        ao            
where Lao is an equivalent air distance from the deflecting means on an optical axis to the exit surface of the single lens, Lbo is a distance from the exit surface of the single lens to the surface to be scanned, Laxcex8 is an equivalent air distance from the deflecting means at an off-axis position to the exit surface of the single lens, and Lbxcex8 is a distance from the exit surface of the single lens to the surface to be scanned.
In the present invention, it is preferable that the power of the exit surface in the sub scanning direction or/and the power of the incident surface in the sub scanning direction changes without correlation to a shape in the main scanning direction.
In the present invention, it is preferable that a radius of curvature of the exit surface in the sub scanning direction changes from an on-axis position to an off-axis position.
In the present invention, it is preferable that the incident and exit surfaces are anamorphic surfaces.
In the present invention, it is preferable that a shape of the incident surface in the sub scanning direction is flat or arc, and a shape of the exit surface in the sub scanning direction is arc.
In the present invention, it is preferable that a distance L from a deflection point where the light beam from the light source means is deflected by the deflecting means to the surface to be scanned satisfies 1.2f less than L less than 1.45f where f is a focal length of the imaging optical system.
In the present invention, it is preferable that there is no inflection point in a curvature change on a surface of the lens shape of the imaging optical system in the main scanning direction, which uses an aspherical surface.
In the present invention, it is preferable that the imaging optical system is manufactured by plastic molding.
In the present invention, it is preferable that a multi-beam laser is used as a light source of the imaging optical system.
In the present invention, it is preferable that the power of the exit surface in the sub scanning direction satisfies 0.95xe2x89xa6xcfx86s2/xcfx86sxe2x89xa61.05 where xcfx86s is power of the overall imaging optical system in the sub scanning direction and xcfx86s2 is power of the exit surface in the sub scanning direction.
In the present invention, it is preferable that the non-arc shape of the exit surface in the main scanning direction satisfies       0.95    xc3x97                  L        bo                    L        ao              ≤            L              b        ⁢                  xe2x80x83                ⁢        θ                    L              a        ⁢                  xe2x80x83                ⁢        θ              ≤      1.05    xc3x97                  L        bo                    L        ao            
where Lao is the equivalent air distance from the deflecting means on the optical axis to the exit surface of the single lens, Lbo is the distance from the exit surface of the single lens to the surface to be scanned, Laxcex8 is the equivalent air distance from the deflecting means at an off-axis position to the exit surface of the single lens, and Lbxcex8 is the distance from the exit surface of the single lens to the surface to be scanned.
In the present invention, it is preferable that the shape of the exit surface of the single lens is an arc shape.
In the present invention, it is preferable that the shape of the incident surface of the single lens within a sub scanning section is flat.
Further object of the present invention is to provide an image forming apparatus comprising the above-described light scanning device, a photosensitive device placed on the surface to be scanned, a developing device for developing an electrostatic latent image formed on the photosensitive body by a light beam scanned by the scanning optical device as a toner image, a transferring device for transferring the developed toner image onto a transfer medium, and a fixing device for fixing the transferred toner image on the transfer medium.
Further object of the present invention is to provide an image forming apparatus comprising the above-described light scanning device, and a printer controller for converting code data input from an external device into an image signal, and inputting the signal to the scanning optical device.
Further object of the present invention is to provide an image forming apparatus comprising a plurality of the above-described light scanning devices, each of the light scanning devices recording image information on photosensitive devices corresponding to each color.
Further object of the present invention is to provide a light scanning device having an imaging optical system for guiding a light beam emitted from light source means to a deflecting element, and forming the light beam deflected by the deflecting element into an image on a surface to be scanned, characterized in that, the imaging optical system is formed from a single lens formed by a molding process, sectional shapes of an incident surface and an exit surface of the single lens in a main scanning direction are non-arc shapes, power of the exit surface in a sub scanning direction satisfies 0.9xe2x89xa6xcfx86s2/xcfx86sxe2x89xa61.1 where xcfx86s is power of the overall imaging optical system in the sub scanning direction and xcfx86s2 is power of the exit surface in the sub scanning direction, and the non-arc shape of the exit surface in the main scanning direction satisfies       0.9    xc3x97                  L        bo                    L        ao              ≤            L              b        ⁢                  xe2x80x83                ⁢        θ                    L              a        ⁢                  xe2x80x83                ⁢        θ              ≤      1.1    xc3x97                  L        bo                    L        ao            
where Lao is an equivalent air distance from the deflecting means on an optical axis to the exit surface of the single lens, Lbo is a distance from the exit surface of the single lens to the surface to be scanned, Laxcex8 is an equivalent air distance from the deflecting means at an off-axis position to the exit surface of the single lens, and Lbxcex8 is a distance from the exit surface of the single lens to the surface to be scanned.